Morbidity and mortality from childhood asthma have been increasing in all developed countries over the past three decades, including in the United States. Numerous theories have been advanced to explain this asthma epidemic, but no single theory has held up to careful scrutiny. Recent international studies have suggested a relatively strong causal relationship between increased risk of childhood asthma and exposure to antibiotics during childhood, especially during the first year of life. The increased asthma risk was seen whether antibiotics were used to treat respiratory or non-respiratory infections. While these previous studies are suggestive, there are significant methodologic concerns about each study. A major concern with most of the studies is their reliance on retrospective recall of antibiotic exposure data from parents years after the exposure. We have data from a prospective, NIH-funded study of the relationship between early environmental exposures and the development of asthma in a birth cohort of children followed to an average 6.7 years of age. At 6.7 years, 482 (58%) of the original 833 children were clinically examined as part of this Childhood Asthma Study (CAS). In addition to clinical histories, the 6- to 7- year clinical examination included skin tests, IgE antibody tests, pulmonary function tests and methacholine challenge. At entry all of the CAS children were within the Health Alliance Plan (HAP) HMO. The current proposal is based on combining the CAS data set with pharmacy data extracted from the HAP data archives. This will allow us to examine possible relationships between antibiotic use, as determined by prescriptions filled, and asthma at 6 to 7 years of age. While not strictly a prospective study, these methods will avoid many of the potential sources of bias found in previous studies. We will also be able to evaluate any relationships between antibiotic exposure and asthma for confounding by other risk factors such as bedroom allergen levels, pet ownership, cigarette smoke exposure, and parental history of asthma or allergy. The proposed study is entirely separate from the goals of the original grant which did not consider antibiotic use as a potential risk factor for asthma or allergy. This new analysis will allow a much more rigorous examination of the possible relationship between early antibiotic use and asthma in a population of American children.